Abstract

Abundances of macrobenthic species were monitored twice yearly (March and September) at 6 locations in Tees Bay, UK, between 1973 and 1996, and once yearly at 4 stations in the outer Tees estuary and 7 stations in the inner estuary between 1980 and 1999. In the Bay, multivariate analysis revealed a serial pattern of community change over years for all areas, but with a major shift in community composition between 1986 and 1988. Inter-annual variability in community composition was significantly greater after 1987 than before 1987 in all areas. Overall, inter-annual variability was greater in areas near the estuary mouth than in areas farther away, although the direction of community change and the timing of the discontinuity were the same in all areas. The serial nature of community change with time was also weaker in the areas close to the estuary mouth. Although there was no clear pattern of change in the number of species present over the sampling period, a dramatic increase in Shannon diversity (H¹) occurred after 1987, due to an increase in evenness that resulted from the reduction of a few previously dominant species, notably the small polychaete Spiophanes bombyx. Although biodiversity measures describing the taxonomic breadth of the species assemblages also showed a marked step change in 1987, this was one of reduced diversity, with average taxonomic distinctness (Δ+) decreasing and the variation in taxonomic distinctness ( Λ+) increasing. These abrupt, detrimental changes coincided with a well-documented change in a variety of components of the North Sea ecosystem during the same period. Traditional species diversity measures, such as H¹, therefore gave a false impression of improving environmental quality over this period: given that the average taxonomic spread was reduced, certain taxa were under-represented with respect to others, and community composition as measured by a multivariate stability index (MSI) became less stable. H¹ also failed to distinguish putatively impacted areas close to the estuary mouth compared with those more distant, despite clear differences in Δ+, Λ+, and in community stability (MSI). Overall patterns of biodiversity and community composition in the Bay have thus been affected temporally by regional changes in the North Sea ecosystem, and spatially by the effects of the estuarine outflow. In the estuary itself, multivariate analysis also revealed a serial pattern of community change, with a major shift in composition in 1994 in both the outer and inner estuary which coincided with the construction of a barrage in the estuary. The numbers of both individuals and species began to increase at this time in the outer estuary. H¹ showed no obvious changes over the period, but in the outer estuary a step change in Δ+ and Λ+ occurred at the same time as that in the Bay. However, the direction of change was the reverse of that in the Bay, suggesting an improvement in environmental quality or a shift to more saline conditions.